Figure 1.

Oxytricha trifallax maintains functionally distinct genomes. (a) The ciliate O. trifallax. (b) The germline micronuclei (red circles) have chromosomes with large blocks of intergenic sequences (red lines) and coding sequences interrupted by IESs (red bars). During differentiation of the macronucleus (green circle), these intergenic sequences and IESs are removed from the large micronuclear chromosomes and the whole genome is fragmented into gene-size nanochromosomes. The ends of the nanochromosomes are stabilized by de novo addition of short telomeric sequences composed of G₄T₄ repeats. (c) A germline chromosome segment harboring a TBE element (blue box) in an intergenic region that will be eliminated during macronuclear differentiation. An enlargement of the TBE structure reveals the 20 bp of telomeric sequence flanking the element. Triangles represent 78-bp inverted repeats and the large blue boxes represent the three ORFs, including the transposase, that are under purifying selection [12].